This invention relates in general to a diffraction grating and more particularly to a process for producing such a grating.
Electromagnetic radiation at any wavelength in the visible and near visible region of the spectrum has a tremendous capacity for transmitting information because of the extremely high frequency of the radiation. This capacity can be increased still further by multiplexing. For example, several beams of light, each of a different wavelength and modulated such that it alone is transmitting a large amount of information, may be combined at a multiplexing device and transmitted as a single beam. Various transmission mediums are available, with single mode optical fiber perhaps being the most useful for high data rates. Of course, to extract the information, the individual beams that form the multiplexed beam must be separated, and this requires a demultiplexer. A diffraction grating is suitable for this purpose, provided the spacing between the grooves of the grating varies. This causes the different wavelengths of the light beam to diffract at different locations along the grating. Thus, each of the beams can be monitored at its own location along the grating.
Holographic procedures are currently used to produce diffraction gratings on glass and other substances as well. Normally a thin layer of glass is deposited on a substrate, and then a layer of photosensitive material, called photoresist, is applied to the surface of the glass. Next, interfering coherent beams of monochromatic light are projected onto the photoresist to produce the image of a grating pattern. This exposes the photoresist which is subsequently developed to dissolve and wash away the exposed areas, leaving the photoresist with a series of alternate ridges and grooves. The grooves are, in effect, transferred to the glass layer by ion-milling which erodes both the photoresist and the glass.
The efficiency of any diffraction grating as a means for separating light of different wavelengths depends to a large measure on the depth of the grooves in the grating. Present optical irradiation procedures do not produce very deep or consistent grooves because the photoresist is not exposed uniformly. In particular, the incident light exposes the photoresist on its surface and generally uniformly throughout its depth because the photoresist is transparent. However, the incident light then reflects from the glass waveguide, as well as from the substrate, and these reflections create standing wave patterns in the photoresist. This secondary exposure destroys the uniformity of the initial exposure produced by the incident light. The distortion is such that upon subsequent development of the photoresist, the closely spaced ridges that remain are deeply undercut (FIG. 6). Indeed, the ridges are so weakened by the undercuts that they cannot withstand the development, and as a consequence they break off and wash away close to the glass film. This in turn limits the depth of the grooves that are produced during the subsequent ion-milling.
Photoresists which are less transparent to the interfering rays would of course greatly reduce the intensity of the standing waves, but these photoresists are formed from inorganic materials and are not as easily processed as organic photoresists, nor are they exposed in a uniform manner throughout their depth by the incident light.